The following account of the prior art relates to one of the areas of application of the present invention, hearing aids.
The acoustic leakage from the receiver to the microphone of a hearing aid (in particular such hearing aids where microphone and receiver are located at a short distance from each other) may lead to feedback instability or oscillation when the gain in hearing aid is increased above a certain point. The condition for instability is given by the Nyquist criterion that provides that oscillation will occur at any frequency where the phase change around the loop is a multiple of 360 degrees AND the loop gain is greater than 1.
In traditional feedback cancellation algorithms it is attempted to model the acoustic feedback path by an adaptive filter and then creating an estimate of the feedback signal. There are several methods to update the adaptive filter.
One commonly used method is to use the output signal (from a processing unit to a receiver) as reference signal and the residual signal after cancellation (of an input signal from a microphone) as the error signal, and use these signals together with an update method of the filter coefficients that minimizes the energy of the error signal, e.g. a least mean squared (LMS) algorithm. This arrangement is termed ‘the direct method of closed loop identification’ and illustrated in FIG. 4 in a hearing aid.
A benefit of the direct method is that the use of a probe noise signal in the reference signal is not necessary provided that the output signal is uncorrelated with the input signal. However, unfortunately in hearing aid applications the output and input signals are typically not uncorrelated, since the output signal is in fact a delayed (and processed) version of the input signal; consequently, autocorrelation in the input signal leads to correlation between the output signal and the input signal. If correlation exists between these two signals, the feedback cancellation filter will not only reduce the effect of feedback, but also remove components of the input signal, leading to signal distortions and a potential loss in intelligibility (in the case that the input signal is speech) and sound quality (in the case of audio input signals).
US 2007/0076910 A1 deals with a method of operating a hearing device system comprising first and second hearing devices located at opposite ears of a person, wherein the microphone signal of each hearing device is wirelessly transmitted to the other hearing device and processed there to reduce the risk of acoustic feedback from a receiver to a microphone of a given hearing device.
WO 99/43185 A1 deals with a binaural hearing aid system comprising first and second hearing devices located at opposite ears of a person, wherein the microphone signal of each hearing device is wirelessly transmitted to the other hearing device, and wherein each hearing aid device comprises signal processing means, which process the microphone signal from its own microphone as well as the microphone signal wirelessly received from the other hearing aid device.